Electroluminescent lamp devices utilize electroluminescent emitting phosphor materials which produce light when a suitable AC electric field is applied thereto. Various structures of, and methods for fabricating, such devices are known to the art. In a typical device, for example, a glass or plastic substrate is coated with a transparent or translucent conducting film. Exemplary films include thin metallic layers, such as gold or silver, or certain semiconducting oxides, such as stannic oxide, which may be doped with antimony, or indium oxide doped with tin. A layer comprising an electroluminescent phosphor, such as zinc sulfide doped with copper and dispersed in a polymeric binder, is then deposited on such film. One or more dielectric layers, such as a barium titanate pigment dispersed in a polymeric binder, or unpigmented resins are commonly used for this purpose. Finally, a conductive metallic layer, such as silver paint or vacuum deposited aluminum, is applied to such structure to form the device. In alternative embodiments such layers may be applied in reverse order and, in such cases, the substrate is commonly aluminum foil.
One technique for depositing such electroluminescent phosphor layer is to deposit such phosphor in a purely random fashion, such as by screen printing, spraying or doctor blade coating techniques.
Although useful electroluminescent lamp devices can be fabricated using such randomly deposited phosphors, such devices often have undesirable properties which arise from particle agglomeration or clustering and as a result of the mixing and coating processes. Thus, the phosphor particles often have such masses that they settle out of solution and such settling action, for example, produces a sparse phosphor population density in some regions or variations in population density from region to region arising from the mixing and coating operations, some regions, for example, containing much thicker phosphor particle layers due to the particle agglomeration. Accordingly, a substantially non-uniform particle distribution exists throughout the mixture and often many, sometime relatively large, regions thereof are completely void of phosphor particles. The resultant coating thereof, when viewed in cross section, is seen to be highly non-uniform in thickness. When an electrode material is deposited onto such structure the electrode layer has non-uniform surface characteristics. When an AC electric field is applied thereto, a substantially non-uniform electric field is produced across the structure and results in both luminous inefficiency in the operation of the device and substantial non-uniformity in brightness over the surface thereof.
Other methods of depositing the electroluminescent phosphor layer attempt to do so in a controlled, non-random, manner usually aimed at producing a single layer, or monolayer, of phosphor particles as by using dusting techniques. Such a method has been described, for example, in the article "The New Phenomenon of Electroluminescence and Its Possibilities for Investigation of Crystal Lattice", G. Destriau, Philosoohical Mag., Vol. 38, (1946/1947).
A similar approach has been described more recently in Japanese Publication No. 27660/1965, Dec. 7, 1965, of Nippon Columbia Co., Ltd. Such publication discloses the use of a thin and uniform fluorescent substrate layer in which the fluorescent powder is arranged nearly in one line and one layer. The layer is formed by applying powdered fluorescent particles to a layer of a high molecular dielectric substance having adhesive properties. The latter layer is used to coat a conductive glass. The fluorescent powder is then pressed down into the dielectric so as to cut into the dielectric layer to form a layer of fluorescent powder generally at the lowest level of the dielectric. The powder forms effectively only one line and one layer thereof and excess fluorescent powder is then removed. The adhesive property of the dielectric is stabilized by curing a further layer, such as a solution of acetone and cyano celluose, which is applied over the fluorescent powder/dielectric layer. Following drying thereof a conductive electrode layer is deposited thereon.
In practicing such monolayer formation techniques, practical problems are encountered which generally have prevented the realization of the intended benefits thereof. Much of the difficulty is due to the process of working the phosphor particles without any further treatment thereof to render it amenable to a dusting process. Since such particles retain a tendency to agglomerate and form clumps or clusters thereof, there is a tendency not to be able to form an effective monolayer. Furthermore, such phosphor layer contains a broad distribution of particle sizes and produces a device which does not exhibit a desired high level of luminous efficiency and a desired uniformity in brightness as is usually required in practical applications. Moreover, when the fluorescent powder is pressed down into the dielectric layer, phosphor particles are frequently brought into contact with the bottom conductive layer and produces electrical problems, i.e., electrical short circuiting.